The CAN (Controller Area Network) protocol, which is frequently used in today's automobiles, will reach its limits in the foreseeable future. The CAN protocol has weaknesses in real-time requirements and the safety relevance of emerging x-by-wire applications in particular. The FlexRay protocol has been developed by a consortium to overcome these restrictions with regard to the special requirements in a motor vehicle. FlexRay is a serial deterministic and fault-tolerant bus system, which meets the increased demands of future networking, in particular higher bandwidths for data transmission, real-time capability and fault tolerance (for x-by-wire systems) in a motor vehicle.
The main requirements considered in the development of FlexRay were a high data transmission rate, deterministic communication, a high fault tolerance, and flexibility. In the FlexRay protocol, information is transmitted in time slots of successive communication cycles. There is a shared understanding of time in the electrical components (network nodes), with the components being synchronized by reference messages (so-called SYNC messages) within a cycle. To allow both synchronous and asynchronous message transmission, the communication cycle is subdivided into a static segment and a dynamic segment, each having at least one time slot (or slot).
The slots of the static segment are assigned to certain messages which are transmitted periodically at certain points in time without competition for the access rights to the FlexRay databus structure. In the static segment of the cycle, the FlexRay databus structure is accessed according to the time division multiplex access (TDMA) method, while during the dynamic segment of the cycle, the bus structure is accessed according to the so-called flexible time division multiple access (FTDMA) method. A so-called mini-slotting method is used for access to the databus structure during the dynamic segment of the cycle. Specification v.2.1 is in effect for the FlexRay protocol. Details of the protocol that might be described here might thus change in the future. Additional information about the FlexRay protocol is available on the Internet at http://www.flexray.de, where several publications have been made accessible to the public.
In closer consideration of a motor vehicle, a networking of various systems having different properties is discernible. Distributed regulating systems usually require cyclical, chronologically synchronized transmission. However, all data not needed constantly, e.g., from vehicle body and comfort applications and/or diagnostic data, may be transmitted asynchronously more advantageously. To allow a deterministic method, the transmission scheme in FlexRay is organized according to cycles. Each communication cycle is divided into a static segment and a dynamic segment of a configurable length. Within the static segment, certain time windows (slots) are allocated to each network node, the messages of the subscriber being transmitted at fixed points in time within these slots. The defined transmission times of the allocated slots ensure a deterministic transmission of data. Bandwidth is allocated in a priority-controlled manner in the dynamic segment of the cycle, i.e., transmission of high-priority messages within one communication cycle is ensured, whereas the point in time of transmission may be delayed for lower-priority messages.
Real-time-relevant and time-critical messages are preferably transmitted in the static segment. The dynamic segment, however, is more suitable for transmission of data having lower real-time requirements. Since these data need not be transmitted in each communication cycle, the network nodes may jointly utilize the available bandwidth in the dynamic segment. The total bandwidth demand is therefore lower; faster repeat rates may be achieved or the baud rate may be reduced by shortening the communication cycle.
Purely static and/or purely dynamic operation is/are also possible owing to the flexibly defined limits between static and dynamic, which opens a broad spectrum of use for FlexRay. In purely dynamic operation, the communication cycle is usually initiated cyclically by a master node. FlexRay also offers the option of initiating the communication cycle through an external event (event triggered).
An alternative communication medium (physical layer) has been developed specifically for this novel protocol. This provides integrated transmission of data and energy on a shared medium. This pertains to high-data-rate powerline communications (HDR-PLC). In contrast with traditional PLC systems from the household field, which rely on an existing infrastructure, a modified (or conditioned) cable tree is used in the case of HDR-PLC, providing an almost ideal transmission function. This allows an inexpensive transceiver structure because a complex receiver structure (e.g., equalizer) may be omitted.
DE 101 42 409 describes powerline communications in a motor vehicle. The structure and embodiment of a conditioned cable tree are described there in particular. DE 101 42 408 describes the use of powerline communications as a redundant databus. DE 10 2004 008 910 describes the use of powerline communications for redundant information transmission in a FlexRay communication system.